Polysaccharides are natural water soluble polymers having saccharides as repeating units forming them. Since polysaccharides show high viscosity when they are formed into aqueous solution, it is difficult to maintain a high content of polysaccharide contained in a composition, such as lotion, ointment, cream or solution. Such high viscosity makes it difficult to ensure homogeneous mixing of the mixture in a composition. In addition, in the case of a composition to be transplanted to a biotissue, biodegradability is very important. Materials to be transplanted to a biotissue should be non-toxic, and are required to be decomposed and discharged by metabolic actions in vivo after the completion of a desired function. It is known that hyaluronic acid, a typical polysaccharide, is subjected to metabolism and discharged at most 3-4 days after it is implanted or transplanted, regardless of the molecular weight of hyaluronic acid and concentration of hyaluronic acid in a composition. Such a short period of metabolism of hyaluronic acid in vivo makes it inadequate to apply hyaluronic acid not only as a biorestoration material that requires a period of at least several months for the maintenance in vivo but also as a post-surgery tissue anti-adhesion agent that requires a period of about 7-10 days for the maintenance.
When a composition containing a high concentration of polysaccharide is to be formed, one of the problems to be solved is high viscosity occurring when a polysaccharide is dissolved into water. To solve the problem, some methods have been developed and particular examples thereof include a method for reducing the molecular weight of a polysaccharide, a method for increasing the osmotic pressure in aqueous solution, and a method for forming polysaccharide into fine powder, followed by dispersion.
The cross-linking processes developed in order to convert a polysaccharide that is a water soluble polymer into a water insoluble gel may be classified into finding a novel cross-linking agent, controlling the molecular weight of a polysaccharide, controlling the concentration of a polysaccharide in cross-linking reaction solution, controlling the cross-linking density, inducing non-uniformity in cross-linking density, and modifying the type of chemical bonding produced between a polysaccharide and a cross-linking agent.
A general method for converting hyaluronic acid into a water insoluble gel includes mixing hyaluronic acid and an epoxy-based cross-linking agent with water adjusted to pH 12 or higher, and maintaining the mixture at a temperature of about 40-60° C. for several hours. Such a cross-linking process is advantageous in that it accelerates the reaction between the epoxy-based cross-linking agent and hyaluronic acid, but it is disadvantageous in that heating with strong base solution causes a drop in molecular weight of hyaluronic acid, resulting in production of a finished water insoluble gel having excessively low mechanical strength. To compensate for such a drop in molecular weight of hyaluronic acid caused by heating with strong base solution, high-molecular weight hyaluronic acid having a molecular weight of about 3,000,000 Daltons is used instead of currently used hyaluronic acid having a molecular weight of about 1,000,000 Daltons. However, this is not preferred in view of cost efficiency. Another method for solving the problem caused by heating with strong base solution includes reducing pH of hyaluronic acid solution to 12 or lower, or reducing the reaction temperature to below 40° C. However, such methods including reducing pH or reaction temperature are problematic in that an excessively long time is required for cross-linking reaction or an increased amount of cross-linking agent is required. To overcome the disadvantages of cross-linking reaction of hyaluronic acid proceeding in solution, there has been developed a method including incorporating solid state hyaluronic acid into an organic solvent containing an epoxy cross-linking agent dissolved therein. However, the surface of water insoluble gel obtained by the method shows high cross-linking density but the internal portion thereof shows little or no cross-linking density. Another method has been suggested to overcome the disadvantage of a drop in molecular weight of polysaccharide such as hyaluronic acid occurring in the cross-linking reaction proceeding in strong base solution, and the method includes dissolving hyaluronic acid and an epoxy-based cross-linking agent into weakly basic water or distilled water adjusted to pH 9 or less or acidic pH, followed by drying to room temperature. By virtue of the method, it was possible to obtain a hyaluronic acid gel having a weight, after complete swelling, corresponding to at most 2 times of its dry weight even with a very small amount of cross-linking agent. However, after transplanting the water insoluble hyaluronic acid gel obtained by the method to a murine biotissue, 30% of the water insoluble gel was decomposed merely in seven days. Such a degree of biodegradability is not suitable for industrial fields where a water insoluble gel state should be maintained in a tissue in vivo for a long time of several months or more.